During plasma cutting or plasma welding, an arc burns between a cathode and an anode. By means of impact ionization, there is produced a hot plasma or a plasma beam of a plasma gas having a temperature or more than 20,000 K. During plasma cutting or plasma welding of metal workpieces, the cathode is typically arranged in a processing head, whilst the electrically conductive workpiece to be processed forms the anode.
Owing to the conventionally used angular form of the cathode and the density distribution of the electrical field lines thereby produced, on the shortest connection line between the anode and the cathode, which corresponds to the center axis of the plasma beam, and in the central region of the plasma torch or the plasma beam adjacent to the center axis, the impact rate of the charged particles in the plasma is particularly high. The high impact rate leads to a high temperature and a high electrical conductivity of the plasma. This central region of the plasma torch is significantly constricted as a result of the increased conductivity and stable in terms of its shape. As the radial distance from the central region increases, the impact rate of the charged particles becomes substantially smaller and the temperature, density and electrical conductivity of the plasma torch decrease. This leads to fluctuations and to an expansion of the plasma beam. This instability and expansion of the edge region of the plasma beam produces an irregular and consequently poor cutting result during a plasma cutting operation. During a plasma welding operation, the weld seam is widened so that a smaller depth can be achieved and a greater thermal distortion occurs.
In order to guide, constrict and stabilize a plasma beam, various approaches are known.
DE 102009006132B4 discloses selective cooling of the plasma beam using a water-cooled nozzle, to draw energy from the plasma at the cathode base location. Owing to the supply and/or discharge of cooling fluid at a right angle relative to the longitudinal axis of the plasma torch head, a significantly longer contact of the cooling fluid with the nozzle is produced at that location. The plasma beam is thereby constricted at the cathode base location.
DE102010005617A1 discloses causing a current which flows through the plasma burner to pulse in a selective or controlled manner at least temporarily during the plasma cutting operation. Inter alia, it is also proposed therein to place the plasma gas and/or a secondary gas in a rotating flow by means of specially formed nozzles of the processing head.
WO2000064618A2 discloses, during plasma welding, superimposing a plasma beam and a laser beam to ignite the plasma beam and to guide it in the laser beam direction. The laser beam serves to excite molecules contained in the plasma gas to vibrations and thus to provide a beam path for the plasma beam.
WO2011029462A1 discloses a device and a method for processing workpieces with an arc device and a laser device in which a laser beam is guided within the plasma gas beam, the laser beam in the plasma gas beam forming a channel for increasing the conductivity of the plasma gas beam. The electrode of the arc device may be constructed as an annular electrode and the laser beam may extend inside the central opening of the annular electrode. Alternatively, one or more laser beam(s) can be directed to the processing location from outside so as to adjoin the plasma gas beam directly upstream of the processing location and so as to intersect it.
DE19944469A1 discloses a device for hybrid welding, in which at least one focused (alternatively optionally also defocused) laser beam is directed onto the workpiece to be processed and an electric arc is produced between an electrode and the workpiece, the axis of the arc being orientated concentrically relative to the laser radiation. The laser beam and the arc strike the workpiece (processing location) substantially at the same location and influence or support each other.